When functioning properly, the human heart maintains its own intrinsic rhythm and is capable of pumping adequate blood throughout the body's circulatory system. However, some individuals have irregular cardiac rhythms, referred to as cardiac arrhythmias, which can result in diminished blood circulation and cardiac output. One manner of treating cardiac arrhythmias includes the use of a pulse generator (PG) such as a pacemaker, an implantable cardioverter defibrillator (ICD), or a cardiac resynchronization (CRT) device. Such devices are typically coupled to one or more conductive leads each having one or more electrodes that can be used to sense bioelectrical cardiac signals and deliver pacing therapy and/or electrical shocks to the heart. In atrioventricular (AV) pacing, for example, the leads are usually positioned in a ventricle and atrium of the heart, and are attached via lead terminal pins to a pacemaker or defibrillator which is implanted pectorally or in the abdomen.
Magnetic resonance imaging (MRI) is a non-invasive imaging procedure that utilizes nuclear magnetic resonance techniques to render images within a patient's body. Typically, MRI systems employ the use of a magnetic coil having a magnetic field strength of between about 0.2 to 3.0 Teslas. The RF fields generated in an MRI environment can induce currents in conductive elements. Currents may be inducted by RF fields in an elongated conductor (e.g., a cable) along an insulated section of a lead and then conducted to a non-insulated element (e.g., a stimulating coil or electrode) of the lead that contacts the patient's tissue. The inducted MRI energy may then convert to heat energy when dissipating to the patient's tissue at the electrode/tissue interface. If high enough in temperature, the heating caused by the dissipating energy may be harmful to the tissue that is adjacent to the lead. The degree of tissue heating is typically related to factors such as the length of the lead, the conductivity or impedance of the lead, and the surface area of the lead electrodes.